Contaminating layers, ranging in thickness from a few Angstroms to perhaps 100 Angstroms, that are present on the surface of an electrically conducting material such as a semiconductor material, a silicide or a metal, are increasingly important in the processing of chips containing integrated circuits. These contaminants may include native oxides on bare silicon or other semiconductor material, native or grown oxides on polysilicon, photoresist smudges on wafers, and organic coatings produced on the wafer through diffusion from other surfaces or through adhesion.
The presence of these contaminating layers often introduces unacceptable uncertainties in performance of the circuits on the chip. For example, the presence of an oxide on a polysilicon surface can drastically interfere with adhesion of subsequently deposited layers such as silicides and can cause peeling of these layers from the adjacent polysilicon. The presence of a native oxide on bare silicon can produce a contact resistance that significantly reduces the electrical current flowing from or to such a layer.
Quantitative information on these contaminants can often be made on bare silicon, providing the locations of the contaminants are known and contaminant thickness exceeds a few tens of Angstroms. However, these measurements, whether obtained through ellipsometry or reflectometry, are quite time consuming because each measurement gives information on a very small illuminated spot. If the contamination is not uniform over the surface of the wafer, it becomes very time consuming to provide an accurate map of the extent of contamination over the entire surface.
Additionally, the surface roughness of the substrate or other layer being examined can mask the presence of thin contamination layers, where ellipsometry or reflectometry is used to provide quantitative measurements. For example, ellipsometry studies must invoke the presence of an "interlayer" of unknown composition to account for the discrepancy between theoretical predictions and experimental results, although these interlayers are never seen in transmission electron microscopy photographs. Further, the sensitivity of such methods tends to decrease when the layers are only a few Angstroms thick because the interference effects are then quite small.
Other methods of examination of characteristics of substrate material that is adjacent to a substrate surface have been disclosed. Smith in Jour. Appl. Phys. 46 (1975) pp. 1553-1558, reviews many of these methods and discloses the use of photoelectron emission from aluminum and from nickel that has or may have an oxide film formed on an exposed surface of the metal. For oxide films, such as NiO, that are photoemitting themselves, photoelectron emission includes current generated within the metal and current generated within the oxide film. For other oxide films, such as Al.sub.2 O.sub.3, that are not photoemitting, photoelectron current emission comes primarily from the metal, with the oxide providing a mask therefor. Most oxides of interest are not photoemitting.
Quiniou et al., in Appl. Phys. Lett. 55 (1989) pp. 481-483, disclose the use of photoemission to perform scanning microscopy of doped regions on semiconductor surfaces. This method, the authors assert, can provide an in situ probe of doping levels or doping patterns immediately below the surface in a semiconductor wafer with a spatial resolution of a few wavelengths of the probing beam. A focused ultraviolet laser beam is scanned across the surface, and differences in surface properties within the wafer material are observed at such differences in emitted photoelectron current. The system disclosed by Quiniou et al. requires a pressure in the electron collector chamber of the order of 10.sup.-3 -10.sup.-1 Pascals and a bias voltage of the order of 70 volts or more and uses a laser beam for illumination of the surface.
The systems used by previous workers for monitoring photoelectron current emitted from or adjacent to a semiconductor surface suffer from a number of defects. Because of the bias maintained between the charged particle collector and the underlying wafer, any small change in distance of separation between the wafer and the collection electrode will induce a capacitive current, which may vary with time. Because the collection electrode is relatively large and the extraction voltages are also large, the induced capacitive current can be significantly larger than the photoelectron current to be measured. It is, therefore, desirable to provide a means of compensating for a capacitive current developed between the wafer and the collection electrode. This compensation means should be capable of monitoring capacitive current developed at any distance of separation of the wafer surface and the collection electrode and should be sufficiently small that the compensation means can be incorporated in the photoelectron current monitoring system.
Another disagreeable feature of photoelectron current monitoring systems of previous workers is that photovoltaic current may also be induced by light beam illumination and thus affect the output signal for current. Photovoltaic current can be induced in a semiconductor material for any photon energy that is appreciably greater than the energy gap E.sub.g for that material, and the energy gap is generally much less than the work function W for the material. For example, the work function or photoelectric threshold for bare silicon ranges from 4.60 to 5.11 eV, depending upon the crystal direction parallel to the incident light beam, but the energy gap in bare silicon is only 1.12 eV. If the photovoltaic current is uniform across the surface of the wafer, its effect on the photoelectron current arising from photoemission might be small and might be eliminated within the system. However, photovoltaic current is affected locally by dislocations and other defects in the semiconductor material
Therefore, it is likely that the photovoltaic current induced will vary from site to site on the surface of the wafer. What is needed is means for compensating for the photovoltaic current at whatever light beam intensity is used. Preferably, this compensation means should be sufficiently small that it may be incorporated in the photoelectron current monitoring system.